This invention relates to peak detector circuits, and more particularly, to a peak detector circuit which has a wide bandwidth and a gain which remains constant with frequency.
Peak detector circuits are widely used, for example, in magnetic disk drives. The read back analog signal from a disk drive head is pulse position modulated. It is desirable to detect the peaks of these pulses to produce a signal which can be used for automatic gain control. The pulse position modulation of the input signal from the disk head is a type of phase or frequency modulation (FM). Any interference which causes phase shift (peak of data pulse shift) degrades the analog signal and introduces timing errors in the resulting decoded data.
An amplitude modulated carrier frequency has no phase error at the carrier zero degrees or 180 degrees points; however, the peak of the AM modulated signal will be displaced from the 90 and 270 degree locations. This peak shift caused by AM modulation applies to an AM modulated FM signal also. Peak shift caused by AM modulation is a source of timing error in a disk drive. AM modulation on the disk head read back analog signal is caused by mechanical structural vibrations which can be minimized but not eliminated.
A wide bandwidth, constant gain, peak detector is desirable for use in an automatic gain control (AGC) loop to remove AM modulation from the data.
When the ratio of the conventional peak detector bandwidth divided by the carrier frequency is a high percentage number (large relatively bandwidth) the peak detector gain changes when the carrier frequency changes. The gain change versus carrier frequency change is proportional to relative bandwidth of the conventional peak detector.
Prior art peak detectors have a trade off of bandwidth versus FM to AM conversion. The larger the relative bandwidth, the greater the FM to AM conversion. It is undesirable to have the change in carrier frequency (FM) cause a peak detector output amplitude variation (AM). This FM to AM conversion by a peak detector causes an error in the extracted AM modulation.
It is an object of the present invention to provide a peak detector circuit having a gain which remains relatively constant regardless of the frequency or the run length of the pulses in the input signal.